linux-kernel - Re: [PATCH v2 04/19] iommufd: Allow pt_id to carry viommu_id for IOMMU_HWPT

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Message-ID: <ac6c371b-857a-42f3-9b71-969ef19a54ba@intel.com>
Date: Fri, 27 Sep 2024 13:38:08 +0800
From: Yi Liu <yi.l.liu@...el.com>
To: Nicolin Chen <nicolinc@...dia.com>
CC: <jgg@...dia.com>, <kevin.tian@...el.com>, <will@...nel.org>,
	<joro@...tes.org>, <suravee.suthikulpanit@....com>, <robin.murphy@....com>,
	<dwmw2@...radead.org>, <baolu.lu@...ux.intel.com>, <shuah@...nel.org>,
	<linux-kernel@...r.kernel.org>, <iommu@...ts.linux.dev>,
	<linux-arm-kernel@...ts.infradead.org>, <linux-kselftest@...r.kernel.org>,
	<eric.auger@...hat.com>, <jean-philippe@...aro.org>, <mdf@...nel.org>,
	<mshavit@...gle.com>, <shameerali.kolothum.thodi@...wei.com>,
	<smostafa@...gle.com>
Subject: Re: [PATCH v2 04/19] iommufd: Allow pt_id to carry viommu_id for
 IOMMU_HWPT_ALLOC

On 2024/9/27 04:10, Nicolin Chen wrote:
> On Thu, Sep 26, 2024 at 04:50:46PM +0800, Yi Liu wrote:
>> On 2024/8/28 00:59, Nicolin Chen wrote:
>>> Now a VIOMMU can wrap a shareable nested parent HWPT. So, it can act like
>>> a nested parent HWPT to allocate a nested HWPT.
>>>
>>> Support that in the IOMMU_HWPT_ALLOC ioctl handler, and update its kdoc.
>>>
>>> Also, associate a viommu to an allocating nested HWPT.
>>
>> it still not quite clear to me what vIOMMU obj stands for. Here, it is a
>> wrapper of s2 hpwt IIUC. But in the cover letter, vIOMMU obj can instanced
>> per the vIOMMU units in VM.
> 
> Yea, the implementation in this version is merely a wrapper. I
> had a general introduction of vIOMMU in the other reply. And I
> will put something similar in the next version of the series,
> so the idea would be bigger than a wrapper.

yep. would be good to see it. Otherwise, it is really confusion what
vIOMMU obj exactly means in concept. :)

>> Does it mean each vIOMMU of VM can only have
>> one s2 HWPT?
> 
> Giving some examples here:
>   - If a VM has 1 vIOMMU, there will be 1 vIOMMU object in the
>     kernel holding one S2 HWPT.
>   - If a VM has 2 vIOMMUs, there will be 2 vIOMMU objects in the
>     kernel that can hold two different S2 HWPTs, or share one S2
>     HWPT (saving memory).

So if you have two devices assigned to a VM, then you may have two
vIOMMUs or one vIOMMU exposed to guest. This depends on whether the two
devices are behind the same physical IOMMU. If it's two vIOMMUs, the two
can share the s2 hwpt if their physical IOMMU is compatible. is it?

To achieve the above, you need to know if the physical IOMMUs of the
assigned devices, hence be able to tell if physical IOMMUs are the
same and if they are compatible. How would userspace know such infos?

-- 
Regards,
Yi Liu